Three patients have been entered into this study thus far and one of these patients was treated with nitrous oxide on two separate occasions. Therefore we have evaluated four clinical situations. The first patient had a maximum period of inhalation of 2 1/2 hours which she tolerated extremely well with no unusual toxicity. The second patient was given an inhalation of 6 hours which she tolerated equally well. The patient who was studied twice tolerated a 40% concentration of nitrous oxide for periods up to 12 hours with only occasional interruption of nitrous oxide inhalation for periods of 1 to 5 minutes. The interruptions were caused by some nausea and vomiting which was eventually well controlled by hydroxyzine. During all trials there was some nausea and mild emesis during study. No patient experienced changes in vital signs or apparent renal, hepatic, or metabolic toxicity. Arterial blood gases remained within normal limits throughout the study for all patients. There were no effects on perpherial white counts or bone marrow examinations in any of the patients. We plan to use higher concentrations of nitrous oxide and longer exposure times in future subjects.